1. Field of Invention
This invention relates to an image forming apparatus utilizing electron-emitting devices and, more particularly to an image forming apparatus in which a spacer as a support member is provided within the apparatus.
2. Description of Related Art
Conventionally, two types of electron-beam emitting devices, namely thermionic cathode electron-beam emitting devices and cold cathode electron-beam devices are known. Examples of cold cathode electron-emitting devices are electron-emitting devices of surface-conduction emission (hereinafter abbreviated to xe2x80x9cSCExe2x80x9d) type, field emission (hereinafter abbreviated to xe2x80x9cFExe2x80x9d) type, and metal/insulator/metal (hereinafter abbreviated to xe2x80x9cMIMxe2x80x9d) type.
A known example of the SCE type electron-emitting devices is described in xe2x80x9cRadio Eng. Electron Phys., 10, 1290xe2x80x9d (1965) by M. I. Elinson, and other examples will be described later.
The SCE type electron-emitting device utilizes a phenomenon where electron-emission is produced in a small-area thin film formed on a substrate, by passing a current parallel to the film surface. As the SCE type electron-emitting devices, electron-emitting devices using an SnO2 thin film by Elinson mentioned above, an Au thin film by G. Dittmer (xe2x80x9cThin solid Filmsxe2x80x9d, 9,317 (1972)), an In2O3/SnO2 thin film by M. Hartwell and C. G. Fonstad (xe2x80x9cIEEE Trans. ED Conf.xe2x80x9d, 519 (1975)), a carbon thin film by Hisashi Araki et al. (xe2x80x9cVacuumxe2x80x9d, vol. 26, No. 1, p. 22 (1983)) are reported.
FIG. 5 is a plan view of the SCE type electron-emitting device by M. Hartwell and C. G. Fonstad described above, as a typical example of the structure of the SCE type electron-emitting devices. In FIG. 5, reference numeral 501 denotes a substrate; 502, a conductive thin film of a metal oxide formed by sputtering, having a H-shaped pattern. An electron-emitting portion 503 is formed by electrification process referred to as xe2x80x9cenergization formingxe2x80x9d to be described later, on the conductive thin film 502. In FIG. 5, the interval L is set to 0.1-1 mm, and the width W is set to 0.1 mm. Note that the electron-emitting portion 503 is shown at approximately the center of the conductive thin film 502, with a rectangular shape, for convenience of illustration, however, this does not exactly show the position and shape of the actual electron-emitting portion 503.
In these conventional SCE type electron-emitting devices by M. Hartwell and the others, the electron emission portion 503 is typically formed by performing electrification, xe2x80x9cenergization-formingxe2x80x9d, on the conductive thin film 502. According to the energization forming process, electrification is made by applying a direct current where voltage increases at a very slow rate of, e.g., 1 V/min., to both ends of the conductive thin film 502, so as to partially destroy or deform the conductive thin film 502, thus form the electron-emitting portion 503 with electrically high resistance. Note that the destroyed or deformed part of the conductive thin film 502 have a fissure. Upon application of appropriate voltage to the conductive thin film 502 after the energization forming, electron emission is made near the fissures.
Examples of the FE type electron-emitting devices are given in, e.g., W. P. Dyke and W. W. Dolan, xe2x80x9cField Emissionxe2x80x9d, Advance in Electron Physics, 8,89 (1956) and C. A. Spindt, xe2x80x9cPhysical Properties of Thin-Film Field Emission Cathodes With Molybdenium Conesxe2x80x9d, J. Appl. Phys., 47,5248 (1976).
FIG. 6 is a cross-sectional view of the FE type electron-emitting device according to C. A. spindt and the others mentioned above, as a typical example of the structure of the FE type electron-emitting devices. In FIG. 6, numeral 601 denotes a substrate; 602, an emitter wiring electrode; 605, an emitter corn; 603, an insulating layer; and 604, a gate electrode. In this device, electron emission is made by applying an appropriate voltage between the emitter corn 605 and the gate electrode 604.
Further, as-another example of the FE type electron-emitting devices, a structure where the emitter and the gate electrode are provided approximately parallel to the substrate surface is known.
Further, examples of the MIM type electron-emitting devices are described in, e.g., C. A. Mead, xe2x80x9cOperation of Tunnel-Emission Devicesxe2x80x9d, J. Apply. Phys., 32, 646 (1961), and other references. FIG. 7 is a cross-sectional view showing a typical structure of the MIM type electron-emitting device. In FIG. 7, numeral 701 denotes a substrate; 702, a lower electrode comprising a metal member; 703, a thin insulating layer having a thickness of about 100 xc3x85; and 704, an upper electrode comprising a metal member having a thickness of 80 to 300 xc3x85. In the MIM type electron-emitting device, electron emission is caused from the surface of the upper electrode 704 by applying an appropriate voltage between the upper and lower electrodes 703 and 702.
In comparison with the thermionic cathode electron-beam emitting devices, the cold cathode electron-emitting devices can obtain electron emission at a lower temperature and, therefore do not need a heater. Accordingly, the cold cathode electron-emitting devices has a structure simpler than that of the thermionic cathode electron-emitting devices, which enables more compact electron-emitting devices. In addition, even if a multitude of electron-emitting devices are arranged on a substrate in high density, heat-melting of the substrate does not easily occur. Further, different from the thermionic cathode electron-emitting devices that have slow response because they operate after being heated, the cold cathode electron-emitting devices have quick response.
For these reasons, the applications of the cold cathode electron-emitting devices have been positively studied.
For example, the SCE type electron-emitting devices have the simplest structure and therefore can be easily manufactured, they are advantageous for forming a large number of electron-emitting devices on a large area. As disclosed in Japanese Patent Application Laid-Open No. 64-31332, many methods for arranging the SCE type electron-emitting devices and driving them have been studied.
Also, applications of the SCE type electron-emitting devices to, e.g., image forming apparatuses such as an image display device and an image recording device, electrical charge beam source and the like have been proposed.
Especially, as applications to image display apparatuses, as shown in the U.S. Pat. No. 5,066,833 by the present applicant, Japanese Patent Applications Laid-Open Nos. 2-257551 and 4-28137, an image display apparatus using the combination of SCE type electron-emitting devices and a fluorescent material which emits light upon reception of an electronic beam has been studied. This type of image display apparatus is expected to have excellent characteristics better than other conventional image display apparatuses. For example, in comparison with recently focused liquid crystal display apparatuses, the above display apparatus is superior in that it does not require a backlight since it is a self light-emitting type and that it has a wide view angle.
Methods for arranging a large number of FE type electron-emitting devices and driving the devices are disclosed in, e.g., the U.S. Pat. No. 4,904,895 by the present applicant. As an application of the FE type electron-emitting devices to an image display apparatus, a flat-type display device is reported by R. Meyer and others (xe2x80x9cRecent Development on Microtips Display at LETIxe2x80x9d, Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6-9 (1991)).
An application of the MIM type electron-emitting devices as an image display device, where a large number of MIM type electron-emitting devices are arranged, is disclosed in Japanese Patent Application Laid-Open No. 3-55738 by the present applicant.
Accordingly, a multi electron-beam source having cold cathode electron-emitting devices wired in a simple matrix has possibilities in a variety of applications. For example, an electron-beam source for an image display device can be provided by appropriately applying an electric signal based on image information to the multi electron-beam source.
Recently, in the field of display devices utilizing cathode luminescence, thin type display devices are desired, and various efforts have been made to reduce the thickness of display devices. For example, as described above, a flat type CRT having a flat-type container where electron-emitting devices are arranged on the rear plate and fluorescent material is arranged on the face plate has been considered. In this flat-type CRT, the weight has been a serious problem.
The flat-type CRT must have an airtight container that maintains a vacuum condition so that electrons emitted from the electron-emitting devices can reach the fluorescent material without colliding with gas molecules in the atmosphere. This prevents the reduction of the weight of the container. Generally, it is preferable to maintain the pressure within the CRT at 10xe2x88x926 torr or less. Since the airtight container must have strength against approximately 1 atm to maintain this inner pressure, it needs thick constituent parts (e.g., face plate, rear plate and side walls), thus the weight of the container is great.
To solve this problem, a structure having support members for supporting atmospheric pressure between the face plate and rear plate has been proposed. This structure obtains sufficient strength even if the thickness of the outer walls (face plate, rear plate and side walls) is thinned.
Thus, the weight of the flat-type CRT can be reduced by providing the support members, however, still there are problems such as the following:
 less than Problem 1 greater than 
The quality of display image is degraded due to electrical charge on the support members.
In the airtight container, there exist many electrically-charged particles, such as ions occurred upon collision of electrons with the fluorescent material, or residual gas particles as well as electrons emitted from the electron-emitting devices. The electrical charge up may occur if these electrically-charged particles continuously collide with the support members.
The electrical charge on the support member changes the electric potential distribution, which disturbs the control of electron beams. For example, the cut-off voltage of the electron beam is drifted, or the electron beam is deflected to traverse an unexpected trajectory. As a result, the degradation of image quality such as disabled luminance control of display images or deformation of image occur.
 less than Problem 2 greater than 
Spark discharge occurs along the surface of the support members. The spark discharge passes a great amount of current through the parts in an instant and damages the fluorescent material and electrodes.
Display devices in which solution of these problems are attempted have been reported.
An example of the display device for solving the first problem is disclosed in Japanese Patent Laid-Open No. 57-118355. FIG. 21 shows the cross-section of the display device, in which numeral 2125 denotes a face plate; 2108, a rear plate; 2123, a fluorescent material; 2113, thermionic cathodes; 2112, support members comprising conductive material, for supporting the thermionic cathodes 2113; 2122, a metal back for applying a voltage to the fluorescent material 2123; 2116 and 2118, electrodes comprising of metal material, for on/off control of electrons emitted from the thermionic cathodes 2113; 2120, electrodes comprising of metal material, for accelerating the electrons; and 2115, 2117, 2119 and 2121, support members comprising of insulating material. The structure, in which electrodes and support members are alternatively layered, supports atmospheric pressure upon the face plate 2125 and the rear plate 2108.
If the support members 2115, 2117 and 2119 are electrically charged, the cut-off voltage of the electron beams drifts to disturb the luminance control of display images. For this reason, the support members are covered with a conductive film. If the support member 2121 is electrically charged, the trajectories of the electron beams are deflected to deform a display image. For this reason, the support member 2121 is also covered with a conductive film.
In this display device, even if charged particles collide with the support members, the electrical charge can be moved through the conductive films to the electrodes and thermionic cathode electrodes, thus electrical charge on the support members can be prevented. As a result, the drift of the cut-off voltage of the electron beams and the deflection of the beam trajectories can be reduced.
A display device in which solution of the second problem is attempted is disclosed in EP 0405262B1.
FIG. 8 shows the cross-section of this display device, in which numeral 801 denotes a face plate; 811, a rear plate; 809, cathodes (FE type electron-emitting devices); 805, fluorescent material; and 803, an anode electrode for accelerating the electrons. The symbol S denotes support members for supporting atmospheric pressure upon the face plate 801 and the rear plate 811. Numeral 813 denotes a side wall of an airtight (vacuum) container.
In this structure, one end of the support member S is in contact with the cathode 809, while the other end of the support member S is in contact with the anode electrode 803, thus the both ends of the support member S receive a high voltage. If the support member S comprises insulating material, spark discharge occurs. However, the spark discharge can be prevented by forming the support member S with conductive material.
Accordingly, this structure can prevent the fluorescent material 805, the anode electrode 803 or the other parts from being damaged by spark discharge.
The above two display devices both provide conductivity to support members. However, the conductivity of the support members electrically connects parts arranged between the support members. To avoid electrical charge up and spark discharge, irregularly-drifting current flows through the support members. In other words, the support members become resources of electric noise. These factors cause the following problems:
 less than Problem 3 greater than 
The modulation of output intensity of electron beams is disturbed. The electrical connection between the support members and the parts in contact with the support members is the main factor of the following troubles:
a. Irregularly-drifting noise intruded into a modulation circuit causes erroneous operation of the circuit. In the worst case, the noise damages the modulation circuit.
b. A modulation signal is leaked to other parts via the support members, causing degradation of image quality such as cross-talk in display images.
c. The load on the modulation circuit increases. In case of a conventional modulation circuit, driving power becomes insufficient due to this increased load, thus lowering the response speed.
For example, in the device shown in FIG. 21, the modulation of electron beams is performed between the electrodes 2116 and 2118. In this structure, irregularly-drifting noise intrudes into a modulation circuit (not shown) connected to these electrodes. Further, the modulation signals applied to the electrodes 2116 and 2118 are leaked to the opposite electrode or to the other parts (e.g., the electrodes 2120 and the thermionic cathodes 2113). Furthermore, the conductivity given to the support members 2115, 2117 and 2119 increases resistive load upon the modulation circuit.
In the device shown in FIG. 8, the modulation of electron beams is performed by applying modulation signals to the cathodes 809. In this structure, irregularly-drifting noise intrudes from the support member S into a modulation circuit (not shown) connected to the cathodes 809. Further, the modulation signal applied to each cathode is leaked to another cathode via the support member S. Furthermore, the conductivity given to the support members S increases resistance load upon the modulation circuit.
 less than Problem 4 greater than 
The operation of the electron-emitting device becomes unstable otherwise the life of the device becomes short. That is, the electrical connection between the support members or parts in contact with the support members causes the following inconveniences
e. Application of irregularly-drifting noise makes the operation of the electron-emitting device unstable. This varies the intensity of emitted electron beams. Further, in comparison with a case where noise does not intrude into the device, the life of the device becomes shorter.
f. Signals applied to other parts are leaked to the electron-emitting device via the support members, and affect electron-beam output to drift. This results in change of the luminance of display images.
For example, in the device shown in FIG. 21, the thermionic cathodes 2113 receive irregular noise from the support members 2115. Further, the signals applied to the electrodes 2116 are leaked to the thermionic cathodes 2113 via the support members.
In the device shown in FIG. 8, the cathodes 809 receive irregular noise from the support members S. Further, if the voltage applied to the anode electrode 803 drifts, the potential of the electron-emitting device is varied due to the drifted voltage.
The present invention has its object to provide an image forming apparatus that solves all the above problems. That is to provide a flat-type image forming apparatus, with support members for the purpose of downsizing of the apparatus, which produces display images without degradation of image quality, and has stable operation, and has a long life.
According to the present invention, the foregoing object is attained by providing an image forming apparatus having: a substrate; an electron-emitting device; a wiring electrode, provided on the substrate, for applying a driving signal to the electron-emitting device; an image forming member to which an electron emitted from the electron-emitting device is irradiated; and an acceleration electrode provided opposing to the substrate, the apparatus comprising:
potential-defining means provided between the acceleration electrode and the substrate;
second support member connected to the potential-defining electrode and the acceleration electrode; and
first support member connected to the wiring electrode and potential-defining means,
wherein the second support member has a semiconductive material surface, and wherein the first support member has resistance greater than that of the second support member by ten times or more, further wherein predetermined potential is applied to the potential-defining means.
Preferably, in the image forming apparatus, the second support member has surface resistivity of 105 [xcexa9/xe2x96xa1] or greater to 1013 [xcexa9/xe2x96xa1] or less.
In the image forming apparatus, an electron-beam source is constituted with the electron-emitting device, connected with m scan-signal wiring electrodes and n information-signal wiring electrodes, layered via an insulating layer therebetween, on the substrate; and the first support member is provided on at least one of the m scan-signal wiring electrodes and n information-signal wiring electrodes; further, the potential-defining means is provided on the first support member.
In the image forming apparatus, the potential-defining means respectively focuses electron beam emitted from the electron-emitting device.
In the image forming apparatus a voltage Vc applied to the potential-defining means satisfies the relations:
0.2xc3x97Qxe2x89xa6Vcxe2x89xa6Q
Q=(Vaxe2x88x92Vf)xc3x97(h+Tc/2)/H
Vc: a voltage applied to the potential-defining means [V]
Vf: a voltage applied to the electron-emitting device [V]
Va: a voltage applied to the acceleration electrode [V]
Tc: a thickness of the potential-defining means [mm]
H: a distance between the electron-emitting device and the acceleration electrode [mm]
h: a distance between the electron-emitting device and the potential-defining means [mm]are satisfied.
In the image forming apparatus, the electron-emitting device is a cold cathode electron-emitting device.
In the image forming apparatus, the electron-emitting device is a surface-conduction emission type electron-emitting device.
In the image forming apparatus, the electron-emitting device is a flat field emission type electron-emitting device.
In the image forming apparatus, the potential-defining means is an ion blocking member which covers above the electron-emitting region of the electron-emitting device.
In the image forming apparatus, the second support members is a plate.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.